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Interpretive Summary: Barley crown rust is a fungal disease and it primarily occurs on barley in the Great Plain regions of the United States. A previous study showed that most of wheat cultivars and lines were immune or highly resistant to this disease except for a few wheat lines with a high level of susceptibility. The objectives of this study were to investigate the genetics of crown rust resistance in two wheat varieties, Chris and Chinese Spring, and determine the chromosomal locations of resistance genes in wheat through genetic and genome mapping analyses. To investigate the genetics of crown rust resistance in wheat, a susceptible winter wheat line PI 350005 was used in crosses with Chinese Spring and Chris. Analysis of hybrids and their progeny populations from these two crosses indicated that crown rust resistance is controlled by one and two genes in Chris and Chinese Spring, respectively. Chromosomal localization of the genes indicated that the gene in Chris, which was designated as Cr1, is located on chromosome 5D and one of the crown rust resistance genes in Chinese Spring is located on chromosome 2D. Genome mapping analysis further positioned Cr1 between two molecular markers located on the long arm of chromosome 5D. This study suggests that crown rust resistance in bread wheat is controlled by one or a few major genes, located on different wheat chromosomes.

Technical Abstract: Barley crown rust, caused by Puccinia coronata var. hordei, primarily occurs on barley in the Great Plain regions of the United States. However, a few genotypes of common wheat were found to be susceptible to this pathogen among 750 wheat accessions evaluated. To investigate the genetics of crown rust resistance in wheat, a susceptible winter wheat accession PI 350005 was used in crosses with two resistant wheat varieties, Chinese Spring and Chris. Analysis of F1 plants and F2 populations from these two crosses indicated that crown rust resistance is controlled by one and two dominant genes in Chris and Chinese Spring, respectively. To determine the chromosome location of the resistance gene Cr1 in Chris, a set of 21 monosomic lines derived from Chris were used as female parents in a cross with a susceptible spring type selection (SSTS35) derived from a PI 350005/Chris cross. Monosomic analysis indicated that Cr1 is located on chromosome 5D in Chris. The results also indicated that one of the crown rust resistance genes in Chinese Spring is located on chromosome 2D, while the other gene is not on 5D and thus it is different from Cr1. Molecular linkage analysis and QTL mapping using a population of 136 doubled haploid lines derived from the Chris/PI 350005 cross further positioned Cr1 between the SSR markers Xwmc41-2 and Xgdm63 located on the long arm of chromosome 5D. Our study suggests that crown rust resistance in common wheat is controlled by one or a few major genes, located on different wheat chromosomes.